Combat aircraft are among the most effective and precise military tools presently available. Because of their fuel consumption requirements, such aircraft are generally configured to carry one or more external fuel tanks. Fuel is drawn from the external tanks until the aircraft reaches the target area, at which time the external tanks are jettisoned. The external fuel tanks thus extend the aircraft's effective range without hampering its maneuverability during combat.
External fuel tanks are judged by several criteria. For instance, the tanks should have an aerodynamic profile which offers minimal air resistance during flight. Moreover, the tank must be designed to offer minimal resistance regardless of the amount of fuel in the tank. Accordingly, many external tanks are prolate, i.e., cigar-shaped.
Some external tanks include a flexible sack which holds the fuel. Such a sack may be distended so that it assumes an aerodynamic profile when initially filled with fuel. However, the surface area of such a conventional sack does not decrease during flight as fuel is consumed. Portions of the sack therefore become unnecessary in that they are no longer needed to form a container with sufficient volume to carry the remaining fuel. As fuel is consumed, increasing excess areas of the sack's surface are therefore freed to flap, ripple, or otherwise distort the aircraft's aerodynamic profile.
Fuel tanks are also judged by their storage and assembly requirements. Because they are often jettisoned after one use, a large number of external fuel tanks may be needed to support a given operation. However, fuel tanks typically hold 300 to 1000 gallons, and therefore require large amounts of storage space unless they can be stored in less volume when they are empty than when they are full. Unless the tanks can be stored compactly, it may be difficult or impossible to find sufficient storage space on an aircraft carrier, for instance, or aboard ships and planes that are used to carry supplies to a distant operation.
The storage requirements of empty fuel tanks are reduced if the tanks are "nestable," that is, if the tanks fit together in a smaller space when they are empty than when they are full. One approach to making tanks nestable is to make each tank an assembly of component sections that fit together in less space than the assembled tanks. Such tanks are herein denoted "sectional" tanks. A typical sectional tank includes a nose section, a tail section, and several center sections. Sectional tanks are designed such that the separated sections nest within one another, thereby reducing the storage space needed for the component sections of empty fuel tanks.
However, the time and effort required to assemble sectional fuel tanks is a major drawback. A typical sectional fuel tank requires several hours of assembly by at least two experienced workers before the tank is ready to receive fuel. If sectional tanks are assembled too far in advance of the time they are needed, the space savings gained by storing the tanks in nested fashion are not realized. But if the sectional tanks are not fully assembled within a few minutes of the time they are needed, necessary sorties and countermeasures by aircraft that rely on external tanks may be difficult or impossible.
External fuel tanks are also judged by their ability to withstand various forces. External tanks are necessarily subjected to strong forces caused by flight maneuvering. In addition, it is not unusual for external tanks to receive forces due to ballistic impact (gunfire), physical impact (crash), or internal fuel ignition. Fuel tanks capable of withstanding such forces, or of degrading gracefully under the impact of such forces, are said to have a degree of survivability.
The survivability of a fuel tank depends to a large extent on the materials used to make the tank. Traditionally, external fuel tanks have been constructed primarily of aluminum or another metal. However, rifle bullets can catastrophically rupture a standard metal tank, essentially causing the tank to explode. Standard aluminum fuel tanks are also vulnerable to fuel fires because aluminum has a relatively low melting point, and to physical impacts because aluminum is brittle.
Composite tanks formed by fiber winding resin-impregnated filaments about a mandrel have superior survivability. Composite tanks leak rather than rupturing when hit by small arms fire. Composite tanks also survive physical impact better than metal tanks because composite tanks are less brittle and have few or no localized seams. In addition, composite tanks can be made with self-insulation, low thermal conductivity, and ablation in order to resist higher temperatures than metal fuel tanks.
Regardless of the material used, however, rigid fully assembled tanks do not readily nest and hence cannot be stored compactly. Fully assembled rigid composite tanks require at least as much storage space as assembled metal tanks of equal carrying capacity. Moreover, the assembly of sectional composite tanks requires substantially the same time and effort as the assembly of sectional metal tanks.
Thus, it would be an advancement in the art to provide a nestable assembled external fuel tank. That is, it would be an advancement to provide an empty external fuel tank which is both highly nestable and substantially ready to receive fuel.
It would also be an advancement in the art to provide such a fuel tank which has better survivability than rigid metal tanks.
It would be a further advancement to provide such a fuel tank which has an aerodynamic profile that offers minimal wind resistance during flight.
It would be an additional advancement to provide a method for forming such a nestable assembled external fuel tank.
Such a nestable assembled external fuel tank and a method for forming such tanks are disclosed and claimed herein.